Control and data transmission system

ABSTRACT

The invention is based on the object of making available a control and data transmission system which uses simple components to permit current-modulated data transmission via a serial bus, and whose components can be used in a flexible way. For this purpose, the control and data transmission system ( 50 ) has a controllable bus terminating device ( 30 ) for impressing a predetermined working current into the bus ( 42, 44 ). Each subscriber ( 1, 2 ) has a D.C.-coupled transmitter ( 60 ) for generating and emitting control signals and data signals which modulate the working current, and a D.C.-coupled receiver ( 20 ) for sensing and evaluating the working current which is modulated by the control signals and data signals.

BACKGROUND OF THE INVENTION

The invention relates to a control and data transmission system, inparticular for use in a motor vehicle, as claimed in the preamble ofclaim 1.

At present, numerous bus systems are known in which a multiplicity ofsubscribers are arranged in series in a serial bus. The data istransmitted from a main device, which is referred to as the bus master,via the bus to all the subscribers which are connected, and then backagain to the bus master. The bus subscribers include, for example,sensors and actuators which can transmit and receive data and controlsignals via the bus. In such bus systems, the data is transmitted eitherby a voltage modulation or by means of an inductively coupled currentmodulation of the carrier signal, it being possible with the latter toachieve better EMC characteristics of the bus system in comparison tovoltage-modulated data transmission if, for example, a twisted line pairis used as bus. Such a bus system is known, for example, from DE-PS 4412 921.

SUMMARY OF THE INVENTION

The invention is based on the object of providing a control and datatransmission system with a serial bus in which the data and controlsignals to be transmitted are current-modulated and whose componentspermit simple and flexible installation.

The invention solves this technical problem with the features of claim1.

To do this, the control and data transmission system has, in addition toa bus which comprises at least one working current loop, a plurality ofsubscribers and a D.C. voltage source, a controllable bus terminatingdevice which impresses a predetermined, essentially constant workingcurrent into the loop-shaped, serial bus. Therefore, the controllablebus terminating device serves to maintain an essentially constantworking current in the bus during the operating state of the control anddata transmission system in that it can compensate, for example,temperature-induced and aging-induced current fluctuations of thesubscribers connected into the bus. The subscribers which can beconnected into the bus each have a D.C.-coupled transmitter forgenerating and emitting control signals and data signals which modulatethe working current, and a D.C.-coupled receiver for sensing andevaluating the working current which is modulated by the control signalsand data signals. Any desired transmission protocols and data formatscan be used to transmit the data and control signals. For example, aspecific subscriber can selectively transmit a message to a selectedsubscriber by virtue of the fact that the data block to be transmittedcontains both the destination address and the source address. Themessage can then be read only by the destination subscriber whosedestination address is contained in the data block.

Advantageous developments are given in the subclaims.

In order to increase the flexibility of the control and datatransmission system and to facilitate its installation, each transmitterhas at least one controlled power source, and the receiver has anammeter. In addition, the receiver ensures that if there is a potentialdifference between the input and output of a corresponding subscriber,said difference is kept essentially constant. At this point it is to benoted that the potential difference is expediently the same in all theconnected subscribers; however, this is not necessary. Each subscriberis also designed in such a way that it can be switched between transmitmode and receive mode. The switching over between transmit mode andreceive mode is carried out, for example, by connecting the transmitterinto the bus in the transmit mode, and the receiver into the bus in thereceive mode.

Because the working current which is to be modulated flows through allthe subscribers, data transmission can take place both in the forwarddirection and in the reverse direction via the bus given an appropriatedimensioning of the controllable bus terminating device.

The flexibility of the control and data transmission system can also beincreased by virtue of the fact that at least one subscriber is capableof serving as a master, that is to say can transmit not only user databut also control data to the other subscribers.

Installation of the control and data transmission system can besimplified by virtue of the fact that each subscriber has a D.C. voltagesupply source and/or a controllable bus terminating device. In this way,it is possible to avoid the use of a separate D.C. voltage source and/orof a separate controllable bus terminating device. Each subscriberinstead contains all the components themselves which are necessary forforming a functional control and data transmission system. Thesubscribers can in addition be manufactured as cost-effective,standardized components by virtue of the fact that they are eachcomposed of at least one, preferably identical integrated circuit. Withsuch implemented subscribers it is possible to disconnect the workingcurrent loop of the bus downstream of any of the subscribers without thebus losing its structure, and thus its functional capability up to thepoint of interruption. This capability is achieved by means of thecontrollable bus terminating device which is integrated in eachsubscriber. If the bus is a two-wire line, the bus terminating device ofthe subscriber which is located in front of the interruption pointcloses the working current loop in the bus.

The bus expediently comprises a two-wire line in the form of a ring lineor spur line.

So that it is ensured that the controllable bus terminating devicecompensates only relatively slow interference fluctuations in comparisonto the data transmission rate, a sufficiently long time constant incomparison to the data transmission rate or bit length is to beselected.

In order to avoid additional lines for supplying power to the individualsubscribers, power is also supplied via the bus.

The efficiency of the control and data transmission system is increasedby virtue of the fact that the system can configure itself after beingput into service or after an interruption, in particular of the bus.

DESCRIPTION OF THE DRAWING

The invention will be explained in more detail below with reference toan exemplary embodiment in conjunction with the appended FIG. 1 which isa schematic representation of such embodiment.

DETAILED DESCRIPTION OF INVENTION EMBODIMENT

The figure illustrates a control and data transmission system 50 whichcan be installed in a motor vehicle, for example. Of course, otherfields of application, for example in automation systems, are alsoconceivable. The control and data transmission system 50 comprises aserial bus with a working current loop, 42, 44 which is a two-wire spurline 42, 44 in the present example. A power supply source 10, referredto below for short as D.C. voltage source, which supplies a D.C. voltageV₀ (for example 20 V) is connected to the left-hand bus end. The D.C.voltage source 10 is followed by a first subscriber 1 to which a secondsubscriber 2 is connected in series. For the sake of a simple method ofrepresentation, the control and data transmission system 50 comprisesonly two subscribers 1 and 2 which are connected in series. Connected tothe right-hand end of the bus 40 is a controllable bus terminatingdevice 30 whose essential function is to impress a predetermined,essentially constant working current into the bus. As illustrated in thefigure, only the conductor 42 of the bus is connected through thesubscribers 1 and 2 and the controllable bus terminating device 30. Theconductor 44 serves as a grounding line or return line to the D.C.voltage source 10.

Each subscriber 1 and 2 has a D.C.-coupled receiver 20 which in turncontains an ammeter 22 which can be connected to an evaluation logiccircuit (not illustrated). The threshold value or values for error-freedetection of current-modulated data can be set precisely at the ammeter22 because the controllable bus terminating device 30 ensures a definedworking D.C. current in the bus 42, 44. The receiver 20 is also designedin such a way that an essentially constant potential difference isapplied between the input and output of each subscriber 1 and 2. Thispotential difference is represented symbolically by the voltage source23 in the receiver 20. It is to be noted that the essentially constantpotential difference which is set between the input and output of eachsubscriber may be the same (for example 0.3 V), but does not have to bethe same. In addition, each subscriber 1 and 2 has a D.C.-coupledtransmitter 60 with a controlled power source 62. Depending on theembodiment, the transmitter 60 can also contain a plurality ofcontrolled power sources in order to generate a digital or analog datacurrent. The power source 62 is designed in such a way that the dataand/or control signals to be transmitted are current-modulated in thetransmit mode. The current can be modulated by virtue of the fact thatthe working current which flows in the bus 42, 44 is varied by apredetermined D.C. absolute value dI with the rhythm of the data to betransmitted. Each subscriber 1 and 2 has, for example, a switch 24which, depending on the operating state of the subscribers, connectseither the transmitter 60 or the receiver 20 into the conductor 42 ofthe bus.

Because, in the present example, the subscribers 1 and 2 are alsosupplied with power via the bus 42, 44, a load resistor 70, which isconnected between the two conductors 42 and 44 of the bus 40, isprovided symbolically in each subscriber 1 and 2. The current flowingthrough the subscribers 1 and 2 via the conductor 42 is thereforereduced by the intrinsic current flowing through the load resistor 70 ofthe subscriber 1 and by the intrinsic current flowing through the loadresistor 70 of the subscriber 2.

Each subscriber 1 and 2 can also have its own controllable busterminating device 30 which can be connected into the circuit. In thisway it is possible for the bus 42, 44 to be interrupted between eachconnected subscriber without the structure and functional capability ofthe bus 42, 44 being adversely affected up to the point of interruption.A further switch 27 is used to connect the bus terminating device 30 ofthe respective subscriber into the bus 42, 44 if the bus is interrupteddownstream of the corresponding subscriber. The subscribers downstreamof the point of interruption can also be connected again into afunctionally capable bus in which a new working current loop is formed.To do this, all that is necessary is to activate the D.C. voltage sourcewhich is arranged downstream of the point of interruption and which canbe implemented in the first subscriber after the point of interruption.If the bus is designed as a ring feeder, after the bus is interruptedeach subscriber can continue to exchange data with all the otherconnected subscribers. To do this, the subscribers upstream anddownstream of the point of interruption must close the bus by means ofthe integrated bus terminating device, and in each case a D.C. voltagesource must be activated at the start of the bus, as a result of whichtwo functionally capable buses are generated as spur lines. As aconsequence, the direction of data transmission is reversed in one ofthe spur lines. In this case it is important for the subscribers to beof symmetrical design with respect to their connection terminals whichconduct working current.

So that the controllable bus terminating device 30 cannot only filterout undesired current fluctuations which are caused, for example, bytemperature-induced and aging-induced influences on the subscribers, andcannot filter out the current-modulated data signals and controlsignals, said bus terminating device 30 is to be dimensioned in such away that the switching-induced time constant is sufficiently long incomparison with the bit length of the data to be transmitted. The timeconstant is, for example, several milliseconds, while the bit length canbe several microseconds. Although it is not shown in the FIGURE, eachsubscriber 1 and 2 can contain in particular a microprocessor forcontrolling the switches 24 and 27 and the controllable power source 62,data signal sources and control signal sources, data signal stores andcontrol signal stores and the like.

In the control and data transmission system 50 illustrated in thefigure, the subscriber 1 operates in transmit mode; this is because thetransmitter 60 is connected into the conductor 42 of the bus via theswitch 24. In contrast, the subscriber 2 operates in receive mode. Inthis case, the receiver 20 is connected into the conductor 42 of the busvia the switch 24. The ammeter 22 of the receiver 20 is configured insuch a way that it can sense the current-modulated data coming from thesubscriber 1 and can feed it to an evaluation device (not illustrated)for further processing.

According to a further embodiment, each subscriber 1 and 2 can have itsown D.C. voltage source 10 so that to completely form the control anddata transmission system 50, in addition to the bus 42, 44, it is onlynecessary to connect the necessary number of subscribers to the bus 42,44. A separate D.C. voltage source 10 and a controllable bus terminatingdevice 30 are then no longer necessary. In addition, each subscriber canfunction as a bus master. Once a subscriber has been selected as busmaster, all the other subscribers operate as what is referred to asslave subscribers.

What is claimed is:
 1. A control and data transmission system, inparticular for motor vehicles, having the following features: a serialbus (42, 44) for transmitting control signals and data signals, aplurality of subscribers (1, 2) which can be connected in series intothe bus (42, 44) and a D.C. voltage supply source (10), characterized bya controllable bus terminating device (30) for impressing apredetermined substantially constant working current into the bus (42,44), each subscriber (1, 2) having; a D.C.-coupled transmitter (60, 62)for generating and emitting control signals and data signals whichmodulate the working current, and a D.C.-coupled receiver (20, 22, 23)for sensing and evaluating the working current which is modulated by thecontrol signals and data signals.
 2. The control and data transmissionsystem as claimed in claim 1, wherein the transmitter (60) contains atleast one controlled power source (62), wherein the receiver (20)contains an ammeter (22) and supplies an essentially constant potentialdifference (23) between the input and output of the subscriber, andwherein each subscriber (1, 2) can be switched between transmit mode andreceive mode.
 3. The control and data transmission system as claimed inclaim 1 or 2, wherein at least one subscriber is capable of serving as amaster.
 4. The control and data transmission system as claimed in one ofclaims 1 or 2, wherein the subscribers can transmit data signals and/orcontrol signals in both directions via the bus (42, 44).
 5. The controland data transmission system as claimed in one of claims 1 to 2, whereineach subscriber (1, 2) has a D.C. voltage supply source (10) and/or acontrollable bus terminating device (30).
 6. The control and datatransmission system as claimed in claim 5, wherein each subscriberincludes at least one integrated circuit.
 7. The control and datatransmission system according to claim 1, wherein the bus terminatingdevice (30) has a time constant longer than the bit length of thecontrol signals and data signals to be transmitted.
 8. The control anddata transmission system according to claim 1, wherein the power issupplied to the subscribers (1, 2) via the bus (42, 44).
 9. The controland data transmission system according to claim 1, wherein eachsubscriber is of symmetrical design with respect to its connectiondevices which conduct the working current.
 10. The control and datatransmission system according to claim 1, wherein the bus comprises atwo-wire line (42, 44) in the form of a ring line or spur line.
 11. Thecontrol and data transmission system according to claim 5, wherein saidcontrol and data transmission system can configure itself by switchingto its respective voltage supply source and controllable bus terminatingdevice in response to degraded circuit parameters or circuitinterruption including bus interruption.